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Article Cite This: ACS Omega 2018, 3, 18489−18498
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Shape Transformation Photolithography: Self-Assembled Arrays of Suspended MEMS Structures from Patterned Polymer Membranes Andriy Sherehiy,† Jeremy M. Rathfon,†,§ Hiroya Abe,†,‡ Sri Sukanta Chowdhury,† and Robert W. Cohn*,† †
ElectroOptics Research Institute and Nanotechnology Center, University of Louisville, Louisville, Kentucky 40292, United States Graduate School of Environmental Studies, Tohoku University, Aramaki 6-6-11-604, Aoba, Sendai 980-8579, Japan
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‡
ABSTRACT: Suspended micromechanical structures are typically formed by dissolving underlying spacer material. However, capillary force-induced collapse during solvent removal can damage soft structures. If instead capillary forces are directed in the plane, they can drive liquid polymeric bridges to directly transform into suspended fibers. The various capillary force-directed methods for fabricating arrays of suspended fibers have suffered from either low manufacturing rates or an inability to produce arbitrary patterns. Shape transformation photolithography (STP) demonstrated herein is a method of producing arbitrarily patterned arrays of suspended fibers that are potentially capable of high fabrication rates. In STP, holes are prepatterned in a polymer nanofilm supported on a micropillar array, and then the film is heated above its glass transition temperature. First, the holes expand by dewetting and then capillary forces drive thinning of the polymer channels defined by the holes. Prepatterning overcomes the energy barrier for hole nucleation and ensures that all fibers form at the same time and with similar diameters. Arrays of fibers and fiber lattice networks are formed from dyed polystyrene films that are patterned with nanosecond laser pulses at 532 nm. The exposure threshold for forming holes is 10.5 mJ/cm2 for single pulses and 3.3 mJ/cm2 per pulse for repetitive pulsing, which is only about 3× larger than the dose available from current 193 nm wafer-stepping projection printers that are used in device manufacture. With the increased absorption of polystyrene at 193 nm and with additional proposed material modifications to the thin film, it may even be possible to employ STP in production wafer steppers at economically feasible manufacturing rates of over 50 wafers/h. single and multidiameter beads.11 Under conditions of ongoing polymerization and slow enough brushing speeds, arrays of suspended membranes form that resemble trampolines, with each membrane anchored between four pillars.12 These various structures can be used directly as micromechanical elements or serve as templates for fabricating suspended structures from other materials. An example of templated fabrication with these structures is that polymer air bridges filled with nanomaterials have been decomposed to produce suspended bridges of nanowires, nanotubes, and graphene.13 Another example is that overcoating air bridges with various materials followed by decomposition or dissolution has been used to fabricate suspended capillaries out of both organic and inorganic materials,2 and overcoating has even been used to template complete, electrokinetically pumped micronscale flow cells.14 Other more exotic multifunctional microsystems that could be built from these processing steps are considered in ref 15. Therefore, compared
1. INTRODUCTION There have been several recent studies on the formation of polymers into patterned arrays of suspended fiber air bridges with diameters from microns down to a few nanometers. These methods include point-to-point fiber drawing from melted1 or solvated polymers,2−5 drawing5 or electrospinning6 over a substrate that is rotating and translating in synchronism with the polymer jet, direct-write from an electrospinning jet in close proximity to the substrate,7,8 and brushing on2 or withdrawing9 solvated polymers from an array of micropillars resulting in an array of suspended fibers. These fiber bridges can have very high aspect ratios of 1000:1 or greater, and correspondingly low bending stiffnesses. Because these structures assemble from material that is suspended at some distance above the substrate, capillary forces (rather than inducing collapse, a common failure mechanism for flexible microstructures that are made by undercut and release using liquid solvents) drive threads of polymeric fluids to thin and solidify into suspended structures. Depending on the polymer concentration, molecular weight and extension rate, fiber geometry due to capillary thinning can vary from near constant diameter10 to bead-on-a-string structures of regularly spaced © 2018 American Chemical Society
Received: October 11, 2018 Accepted: December 17, 2018 Published: December 27, 2018 18489
DOI: 10.1021/acsomega.8b02763 ACS Omega 2018, 3, 18489−18498
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A perspective on the significance of the method can be gained by considering the degree to which this process is compatible with current tools used in the mass production of integrated circuits, specifically, in terms of available optical energy and exposure throughput rate (which is dictated by economic arguments about return on investment). The ratelimiting step for STP, and photolithography in general, is pattern exposure (because the thermal annealing and membrane attachment steps can be performed on an entire wafer, or even a cartridge of prepatterned wafers all at the same time.) We specifically consider compatibility of STP with the widely deployed 193 nm wavelength production steppers that use excimer laser light sources (that produce nanosecond pulses at kilohertz rates) which currently dominate the manufacture of computer processors and memory chips. (Only recently have 13.5 nm extreme UV steppers begun to be used in chip manufacture.) The 193 nm wavelength is strongly absorbed by many organic polymers,20−22 many of which have been employed as photoresists, including polystyrene (PS), which is considered in this report. For STP to be used in production setting, it is necessary that holes be patterned in the polymer films with the fluence available from these steppers (around 0.3−1.1 mJ/cm2, see Table 2 in Section 3). While previous reports and experiments presented herein suggest that the exposure threshold of PS is somewhat higher than the levels available from production steppers, other polymers and modifications to PS are expected to have lower exposure thresholds (which is discussed in Section 3.1).
to traditional lithographic processes of deposition, patterning, and release by undercutting, these various approaches to forming suspended polymer structures offer ways to selfassemble very flexible and delicate structures, in the third dimension above the substrate, and with fewer steps and less likelihood of capillary force-induced snap-down. However, all these methods of directly fabricating fiber air bridges1−9 suffer either from low manufacturing throughput, in the case of serial direct-write methods, or from a limited range of geometries (basically, only periodic arrays) that can be patterned by brush-on methods and synchronized electrospinning. In this report, we introduce shape transformation photolithography (STP), a method to form much more general and arbitrary patterns of fiber bridges. In STP (Figure 1), a micropillar-suspended nanofilm is optically prepatterned, followed by thermal annealing during
2. RESULTS This section presents several experimental demonstrations of STP. The process (documented in Materials and Methods) is summarized in the schematic (Figure 1a). A polymer thin film mounted on a micropillar array is optically patterned with holes by laser exposure. The holes may be simple round holes or more complex patterns (as shown in Figures 1−4). Upon heating to a point where the polymer can flow, the holes expand and transform to threads that bridge pairs of pillars. These threads thin further into fibers of nearly constant diameter over most of their length. In these experiments, PS films and 532 nm lasers (initially, continuous wave and later, nanosecond pulsed) were used. PS absorbs almost no light at this wavelength, so the films are dyed to a level approaching the absorbance of pure PS at 193 nm. The threshold fluence that we find for forming holes at 532 nm (presented below in Figure 5 and Table 1) is close to the reported ablation threshold for PS at 193 nm.23 The dye not only increases optical absorption, but also lowers the glass transition temperature of PS (see Table 1 and Section 3.1 below). The relevant characteristics of the various films used in experiments are presented in Table 1a,b together with calculated optical transmission characteristics of PS films at 193 nm (Table 1c) derived from measurements in ref 22. 2.1. Transformation from Film to Fibers. The location of holes in a micropillar-suspended film determines the pattern of fibers that evolve during the annealing step. The evolution from holes to fibers is shown in Figure 1b. The fibers form by heating the film to between T1 and T2 (Table 1). For the pure PS samples, these empirical temperatures are within a few degrees of the reported glass transition temperature (Tg). At T1, the patterned holes begin to expand
Figure 1. STP: method and demonstration of the transformation of a polymer thin film (the 80 nm thick region of film 6) into patterned arrays of polymer fiber air bridges. (a) Three-dimensional schematic view of the method. (b) Photographs of the film before patterning and at various stages of the thermal anneal as the film transforms from holes into fibers. (c) Wider field of view of the film in (b) during early hole expansion. The unpatterned areas (outside the dotted box) have many randomly nucleated holes. The temperature has just reached 97 °C when the nuclei first appear. In (d) where the temperature has reached 125 °C, the nuclei expand to produce a random pattern of fibers, while the prepatterned areas (inside the dotted box) have transformed into a well-ordered array of fibers in (d).
which the low-resolution patterns transform through capillary thinning into a defined pattern of fiber bridges. Prepatterning of holes overcomes the otherwise uncontrolled nucleation of holes at random times and locations and the undesirable evolution of these holes into fibers of widely varying diameters, orientations, and random breaking times.16−18 Here, we experimentally demonstrate that the prepatterned films can be formed by photolithography, which enables, for the first time, a parallel method of capillary force-directed self-assembly that produces arbitrarily defined patterns of suspended fibers. 18490
DOI: 10.1021/acsomega.8b02763 ACS Omega 2018, 3, 18489−18498
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Figure 2. Evolution of a prepatterned film (film 6, 80 nm region) into suspended fibers. Each photograph (a−h), taken in a chronological order, is annotated with the temperature at the time the photograph was taken. The temperature was either increasing with time or held constant for a specific duration. The hold time at which a photograph is taken is also noted. The dark spot to the right of the “UofL” pattern is residue from the destruction of a pillar by unintentionally illuminating it with the laser while prepatterning the film. The fiber-thinning characteristics do not appear to be affected by proximity to the residue. There are additional periods where film-to-fiber transformation appears unchanging. These are 2.5 min between 25 and 95 °C, 8 min between 114 and 120 °C and 2 min between 120 and 126 °C. The scale bar applies to all panels a−h.
Figure 4. Diagonal fibers formed by STP from film 5 (100 nm thick). Prepatterned films (upper row) and films after annealing (lower row) that form diagonal fibers at (a) 45°, (b) both +45° and 45°, and (c) 26.6° from horizontal. The pitch of the (silicon) pillars is 40 μm in (a) and 30 μm in (b,c).
Figure 3. How varying the location of holes in 125 nm thick film 3 (top row of images) leads to different structures forming during annealing (bottom row of images). (a) Holes centered between four pillars produce fibers in x and y. (b) Holes centered between each pair of pillars in a group of four pillars form lattice-like structures. (c) At the junction between hole patterns centered between four pillars, like (a), and centered between two pillars, like (b), additional fiber forms in y that are thinner than the fibers to the left. The scale bar applies to all six panels of the figure.
provides considerable processing latitude for when the heat can be removed and the fiber is allowed to solidify. 2.2. Need for Prepatterned Holes. Figure 1c,d gives a wider field of view of the same experiment from Figure 1b. Here, the film is slowly heated (at 1 °C/min) to help observe the process. Holes are first observed to randomly nucleate at 97 °C in the unpatterned region (outside the red box in Figure 1c.) At 125 °C (Figure 1d), fibers are fully formed with welloriented fibers in the prepatterned region and with randomly oriented networks of fibers in the unpatterned regions. Not only does the prepatterned region have a much lower density of nucleated holes, but the expansion and transformation into fibers of the prepatterned holes appear negligibly affected by the smaller holes. The four fibers that form from the prepatterned holes first reach stable diameters at 125 °C and remain at the same diameter up to the final temperature of 130 °C. In the unpatterned area, individual branches of branched fibers begin to break at 125 °C and most
(by viscoelastic dewetting17,25,26). While the fibers thin to a stable diameter above T1, scattered remnants of the membrane may remain at this temperature. However at T2, these last remnants do flow and absorb into the fibers. As shown in Figure 1b, for a pair of holes patterned on each side of an imaginary line connecting two pillars, the hole edges expand toward each other to form a capillary bridge. In Figure 1b, the four holes define two vertical and two horizontal bridges. The bridges continue to thin, but eventually reach a stable diameter that persists for a considerable period of time (on the order of 25 min at 130 °C for pure PS18) before the fibers show necking or beaded features of Rayleigh plateau breakup. This stability 18491
DOI: 10.1021/acsomega.8b02763 ACS Omega 2018, 3, 18489−18498
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Figure 5. Exposure threshold at 532 nm for dyed PS films 1−3. (a) Resulting hole diameter as a function of fluence for single laser pulses. The inset shows a series of holes in film 3 that increase in diameter with increasing fluence. The numbers identify the corresponding diameter-fluence values on the film 3 graph. (b) Dependence of exposure threshold on the fluence (per pulse) for the given number of pulses that produces the first observed (minimum diameter) hole. Dashed lines in (a,b) are included to guide the eye.
Table 1. Properties of the Pure and Dyed PS Films Studied Herein temperature at which
film
starting solutiona (wt % PS)
1 2 3 4 5
0.25 0.75 1.25 1.75 0.75
6
1.25
film thicknessb (nm)
hole expandsc T1 (°C)
exposure threshold per pulse
film disappears T2 (°C)
transmittance 532 nm T
absorbance 532 nm A = 1 − T − R
single pulse (mJ/cm2)
(a) Exposure: Pulsed Laser; Film: Spun on, Dyed; MW PS: 400 000 (Films 1−4), 45 000 (Film 5) 85 (20) 55 (100) 65 (110) 0.44 0.45 10.5 105 (30) 65 (100) 75 (110) 0.36 0.55 28.1 125 (55) 95 (110) 105 (120) 0.27 0.66 66.7 200 110 120 0.18 0.77 100 45 50 0.37 0.55 11.6 (b) Exposure: CW Laser; Film: Doctor Blade, Gradient Thickness, Dyed; MW PS: 400 000 80 120 125 0.52 0.37 2.6 × 106 120 0.49 0.40 170 0.40 0.50 (c) calculatede optical properties for pure PS thickness transmittance absorbance 193 nm (nm) 193 nm 30 0.16 0.63 55 0.04 0.82 80 0.01 0.84 >120
